With advancement of information technology, information recording technology, particularly magnetic recording technology, has progressed remarkably. In a magnetic disk used for an HDD (hard disk drive) which is one of the magnetic recording media and so on, rapid miniaturization, production of thinner disk, increase in recording density and speedup of access rate have been continued. The HDD performs recording and playbacking while allowing a magnetic disk having a magnetic layer on a discal substrate to rotate at a high rate and allowing a magnetic head to fly floating above this magnetic disk.
Higher substrate strength is demanded for a magnetic disk since the rotary rate of the magnetic disk increases with the increase of access rate. In addition, with the increase of recording density, the magnetic head changes from a thin film head to a magnetoresistive head (MR head), further to a giant magnetoresistive head (GMR head), and further with the introduction of DFH (Dynamic Flying Height) control mechanism, the flying height from the magnetic disk of the magnetic head (the narrowest distance among the gaps between the magnetic head and the magnetic disk) becomes narrower to around 2 nm. On this account, when there are irregularities on the magnetic disk surfaces, there may be caused crash failure due to collision of the magnetic head, thermal asperity failure which leads to read errors due to heat caused by adiabatic compression of the air or contact thereof. It becomes important to finish the main surfaces of the magnetic disk as an extremely smooth surface to suppress such troubles caused on the magnetic head.
Therefore, glass substrates have come to be used lately as substrates for a magnetic disk in place of conventional aluminum substrates. This is because the glass substrates consisting of glass, which is a rigid material, can be superior to the aluminum substrates consisting of a metal, which is a flexible material, in smoothness of the substrate surfaces. In addition, since the glass substrate is harder than the aluminum substrate, distortion and flapping of the substrate at the time of high-speed rotation can be suppressed. Thereby, collision risk against the head can be reduced.
In the meantime, the flying height of the head also decreases with the improvement of the recording density when the glass substrate is used, and therefore surface smoothing of the glass substrate for magnetic disks and the removal of particles (contamination) become still more important. Since magnetic particles are divided respectively in the next generation bit-patterned media and discrete track media in particular, it is expected that presence of fine irregularities on the glass substrate surfaces and fine particles adhering to the glass substrate surfaces become serious. Therefore, a polishing step and a supersonic treatment step are performed on the glass substrate to improve smoothing of the glass substrate and to remove particles on the glass substrate surfaces (for example, refer to Patent Document 1).